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Lorentzova simetrija je jedna od fundamentalnih simetrija u modernoj fizici, ipak, određeni modeli kvantne gravitacije upućuju na mogućnost narušenja iste. Jedan od mogućih načina detekcije narušenja Lorentzove simetrije omogućuje foton–foton raspršenje gama-zrake na ekstragalaktičkom pozadinskom zračenju. U ovom radu analizira se foton–foton raspršenje u kontekstu standardne i proširene teorije kvantne elektrodinamike. Dan je pregled izvoda Fermijevog zlatnog pravila i udarnog presjeka za foton–foton raspršenje u standardnoj kvantnoj elektrodinamici. Promotrena je ovisnost udarnog presjeka uz pretpostavku modificirane kinematike procesa. Pomoću modela proširene kvantne elektrodinamike proučava se utjecaj dinamičkih promjena na udarni presjek za foton–foton raspršenje. Analiziran je izvod udarnog presjeka u limesu visokih energija gama-zrake. Izračunat je standardni udarni presjek u laboratorijskom sustavu i dan je opći oblik udarnog presjeka dvočestičnog raspršenja. Primjenjujući dobivene rezultate, odbacuje se ultrarelativistička aproksimacija čime je račun za modificirani udarni presjek za foton–foton raspršenje precizniji u području svih energija. Proučena je kinematika raspršenja u laboratorijskom sustavu., Lorentz symmetry is one of the fundamental symmetries in modern physics, however, certain models of quantum gravity point to the possibility of its violation. One possible way of detecting Lorentz symmetry breaking is provided by photon–photon scattering of gamma-rays on the extragalactic background radiation. In this thesis, photon–photon scattering is analyzed in the context of the standard and extended theory of quantum electrodynamics. An overview of the derivation of Fermi’s golden rule and the cross section of photon–photon scattering in standard quantum electrodynamics is given. The analysis of the cross section with the modified reaction threshold is done. Using the model of extended quantum electrodynamics, the influence of dynamic changes on the impact cross section for photon–photon scattering is studied. The cross section in the ultra–relativistic approximation is analyzed. The standard cross section in the laboratory system is calculated and the general form of the cross section of two-particle scattering is given. Applying the obtained results, the ultra–relativistic approximation is discarded, which makes the calculation for the modified cross section for photon—photon scattering more accurate in the range of all energies. The kinematics of scattering in a laboratory system was studied.

Selection rules that follow from parity and four-momentum conservation are listed for head-on light-by-light scattering in a strong magnetic field taking into account nontrivial dispersion laws of different photon eigen-modes. The wave-length shifts occur for certain transitions between photon eigen-modes.

Although experimental efforts have been active for about 30 years, a direct laboratory observation of vacuum magnetic birefringence, due to vacuum fluctuations, still needs confirmation: the predicted birefringence of vacuum is $$\Delta n = 4.0\times 10^{-24}$$ @ 1 T. Key ingredients of a polarimeter for detecting such a small birefringence are a long optical path within the magnetic field and a time dependent effect. To lengthen the optical path a Fabry–Perot is generally used with a finesse ranging from $${{\mathscr {F}}} \approx 10^4$$ to $${{\mathscr {F}}} \approx 7\times 10^5$$ . Interestingly, there is a difficulty in reaching the predicted shot noise limit of such polarimeters. We have measured the ellipticity and rotation noises along with Cotton-Mouton and Faraday effects as a function of the finesse of the cavity of the PVLAS polarimeter. The observations are consistent with the idea that the cavity mirrors generate a birefringence-dominated noise whose ellipticity is amplified by the cavity itself. The optical path difference sensitivity at $$10\;\hbox {Hz}$$ is $$S_{\Delta {{\mathscr {D}}}}=6\times 10^{-19}\;\hbox {m}/\sqrt{\mathrm{Hz}}$$ , a value which we believe is consistent with an intrinsic thermal noise in the mirror coatings. Our findings prove that the continuous efforts to increase the finesse of the cavity to improve the sensitivity has reached a limit.

We present the dimensioning of a photon-photon collider based on Compton gamma sources for the observation of Breit-Wheeler pair production and QED $\ensuremath{\gamma}\ensuremath{\gamma}$ events. Two symmetric electron beams, generated by photocathodes and accelerated in linacs, produce two gamma ray beams through Compton back scattering with two J-class lasers. Tuning the system energy above the Breit-Wheeler cross section threshold, a flux of electron-positron pairs is generated out of light-light interaction. The process is analyzed by start-to-end simulations. Realistic numbers of the secondary particle yield, referring to existing state-of-the-art set-ups and a discussion of the feasibility of the experiment taking into account the background signal are presented.

The hadronic part of the electron structure function $F_{2}^{e}$ has been measured for the first time, using $e^{+}e^{-} $ data collected by the DELPHI experiment at LEP, at centre-of-mass energies of $\sqrt{s}$=91.2-209.5 GeV. The data analysis is simpler than that of the measurement of the photon structure function. The electron structure function $F_{2}^{e}$ data are compared to predictions of phenomenological models based on the photon structure function. It is shown that the contribution of large target photon virtualities is significant. The data presented can serve as a cross-check of the photon structure function $F_{2}^{\gamma}$ analyses and help in refining existing parameterisations.

Experimental bounds on induced vacuum magnetic birefringence can be used to improve present photon-photon scattering limits in the electronvolt energy range. Measurements with the Polarizzazione del Vuoto con Laser apparatus [E. Zavattini et al., Phys. Rev. D 77, 032006 (2008)] at both $\ensuremath{\lambda}=1064$ and 532 nm lead to bounds on the parameter ${A}_{e}$, describing nonlinear effects in QED, of ${A}_{e}^{(1064)}l6.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}@1064\text{ }\text{ }\mathrm{nm}$ and ${A}_{e}^{(532)}l6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}@532\text{ }\text{ }\mathrm{nm}$, respectively, at 95% confidence level, compared to the predicted value of ${A}_{e}=1.32\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{T}}^{\ensuremath{-}2}$. The total photon-photon scattering cross section may also be expressed in terms of ${A}_{e}$, setting bounds for unpolarized light of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}^{(1064)}l4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}62}\text{ }\text{ }{\mathrm{m}}^{2}$ and ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}^{(532)}l2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}60}\text{ }\text{ }{\mathrm{m}}^{2}$. Compared to the expected QED scattering cross section these results are a factor of $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ higher and represent an improvement of a factor about 500 on previous bounds based on ellipticity measurements and of a factor of about ${10}^{10}$ on bounds based on direct stimulated scattering measurements.

IIn 2006 the PVLAS collaboration reported the observation of an optical rotation generated in vacuum by a magnetic field. To further check against possible instrumental artifacts several upgrades to the PVLAS apparatus have been made during the last year. Two data taking runs, at the wavelength of 1064 nm, have been performed in the new configuration with magnetic field strengths of 2.3 T and 5 T. The 2.3 T field value was chosen in order to avoid stray fields. The new observations do not show the presence of a rotation signal down to the levels of $1.2\cdot 10^{-8}$ rad at 5 T and $1.0\cdot 10^{-8}$ rad at 2.3 T (at 95% c.l.) with 45000 passes in the magnetic field zone. In the same conditions no ellipticity signal was detected down to $1.4\cdot 10^{-8}$ at 2.3 T (at 95% c.l.), whereas at 5 T a signal is still present. The physical nature of this ellipticity as due to an effect depending on $B^2$ can be excluded by the measurement at 2.3 T. These new results completely exclude the previously published magnetically induced vacuum dichroism results, indicating that they were instrumental artifacts. These new results therefore also exclude the particle interpretation of the previous PVLAS results as due to a spin zero boson. The background ellipticity at 2.3 T can be used to determine a new limit on the total photon-photon scattering cross section of $\sigma_{\gamma\gamma} < 4.5 \cdot10^{-34}$ barn at 95% c.l.., Comment: 25 pages, 7 figures Main changes rel. to v.2: minor changes to abstract, replaced Figures 4,5,6, corrected typographical errors. Paper submitted to Physical Review D

Einstein's theory of general relativity is a cornerstone in the process of gaining increased understanding about problems of gravitational nature. It can be applied to problems on the huge length scales of cosmology and as far as we know it does not break down before the Planck scale is approached. Irrespective of scale, a perturbative approach is often a very useful way to reduce the Einstein system to manageable complexity and size. The projects included in this thesis can be divided into three subcategories. In the first category the keyword is photon-photon scattering. General relativity predicts that scattering can take place on a flat background due to the curvature of space-time caused by the photons themselves. The coupling equations and cross-section are found and a comparison with the corresponding quantum field theoretical results is done to leading order. Moreover, photon-photon scattering due to exchange of virtual electron-positron pairs is considered as an effective field theory in terms of the Heisenberg-Euler Lagrangian resulting in a possible setup for experimental detection of this phenomenon using microwave cavities. The second category of projects is related to cosmology. Here linear perturbations around a flat FRW universe with a cosmological constant are considered and the corresponding temperature variations of the cosmic microwave background radiation are found. Furthermore, cosmological models of Bianchi type V are investigated using a method based on the invariant scheme for classification of metrics by Karlhede. The final category is slowly rotating stars. Here the problem of matching a perfect fluid interior of Petrov type D to an exterior axisymmetric vacuum solution is treated perturbatively up to second order in the rotational parameter.

We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is (3.9+/-0.5)e-12 rad/pass, at 5 T with 44000 passes through a 1m long magnet, with lambda = 1064 nm. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed., 11 pages, 4 figures, submitted to Physical Review Letters Comment to version 2: minor changes to abstract and final discussion. Added 2 references Comment to version 3: corrected minor typographical errors, eliminated the distinction between scalar and pseudoscalar in the particle interpretation of the result